Electric Motor

Abstract

An electric motor used for example in a magnetic disk drive, comprises a rotor having a hub on which the magnetic disk is mounted, and an annular driving permanent magnet, and rotating together with a rotary shaft, and a stator disposed outside the rotor, having a core and coils wound on the core. A signaling permanent magnet for detecting the rotary angle or the rotational speed of the rotor may be provided on the radially inner side of the driving permanent magnet. Part of the outer periphery of the stator core may be retracted radially inward or cut away to form a concavity or an open part. A magnetic shielding plate may further be provided to cover the coils, with part or the entirety of the magnetic shielding plate being positioned closer to the rotary shaft than the outermost periphery of the rotor to prevent the escape of the rotor in the direction of the rotary shaft. The stator core may be provided, at its inner extremities, bent portions extending in the direction parallel with the rotary shaft, and away from the hub. Alternatively, the midpoint of the stator core may be positioned farther away from the hub than the midpoint of the driving permanent magnet.

What is claimed is: 
     
       1. An electric motor comprising: a rotary shaft that is rotatably supported;   a rotor having an annular driving permanent magnet generating a magnetic flux in the radial direction, and rotating together with said rotary shaft the rotor further comprises a hub fixed to said rotary shaft;   a stator disposed outside the rotor, having a core and a plurality of coils wound on the core, and generating a rotating torque for the rotor by the interaction with the driving permanent magnet;   means for detecting the rotary angle or the rotational speed of the rotor,   said means comprises a signaling permanent magnet having a length less than the length of the driving permanent magnet, provided on only a lower radially inner side of the driving permanent magnet at an end farther away from said hub wherein said signaling permanent magnet is formed by magnetization in the axial direction; and   said means further includes an FG detecting circuit formed on a circuit board spaced with a gap from the signaling permanent magnet to interact with the signaling permanent magnet.   
     
     
       2. The electric motor of claim 1, wherein said core comprises an annular part and tooth portions extending radially inward from the inner surface of the annular part, and said coils are wound on the tooth portions. 
     
     
       3. The electric motor of claim 2, wherein all said tooth portions are bent. 
     
     
       4. The electric motor of claim 1, wherein said driving permanent magnet has alternate magnetic poles on the outer surface thereof to face the inner surface of the core. 
     
     
       5. The electric motor of claim 1, wherein said rotor further comprises an annular yoke fixed to said hub; and said annular permanent magnet is mounted on the outer surface of the annular yoke.   
     
     
       6. The electric motor of claim 5, wherein said signaling permanent magnet has poles on its end farther away from said hub. 
     
     
       7. The electric motor of claim 1, wherein said shaft is rotatably supported for movement by ball bearings.